Flip-chip packaging was introduced by IBM 30 years ago, and has been used for high-end devices and niche market-oriented up to now.
Flip-chip means not a chip (or cell) or a package type with particular specifications but a method connecting dies to carriers electrically in relation to electrodes. The method using wires for connection is called wire-bond packaging. Flip-chip packaging can be applied to both mesa chip type and vertical chip type.
A die with conductive bumps is flipped and directly connected to a carrier in case of the flip-chip type. Heat generation capacity is thus much bigger than that of wire bond package.
In case of most of electronic devices such as ASIC, microprocessor and SOC (System On Chip), the power consumption for heat generation is about 10-25 W, which is 5-10 W higher than the power consumption which is processed with high temperature wire bond ball grid array (BGA).
On the other hand, flip-chip package power consumption capacity according to heat dissipation design considering heat requirements (maximum junction temperature, ambient temperature and air flow), and package parameters (external cooling structure, package/die size, the number of circuit board layers, the number of balls, etc) is usually 25 W.
Excellent heat dissipation capacity of flip-chip package is due to the structure having heat dissipation parts. In this case, heat is dissipated through thermal balls and internal/external heat dissipation parts. In addition, Flip-chip package need not wire bonding which behaves as a bottle neck of heat; therefore, the package shows good electric performances.
The technology of flip-chip package method as described above has been particularly used to improve the light emitting efficiency of LED in addition. This technology is called as ‘leadless semiconductor’ because the electrode patterns of a chip on the side to face a circuit board is fused directly with the corresponding patterns of the board without intervened medium such as BGA (ball grid array) or additional connecting structures related metal lead (wire) when semiconductor chips are attached to the board. In addition, the size of the package can be reduced to that of a chip or chips as small as possible; therefore, it is easier to make the package smaller and lighter, resulting in more finite pitch (a distance between electrodes).
Light is usually extracted from a surface of Nitride semiconductors which are generally grown on a substrate of sapphire, insulator. No good thermal conductivity of the sapphire substrate has been regarded as a major problem in heat dissipation. Flip-chip technology was suggested to solve the problem accordingly. The technology is packaging electrodes of cells on PCB (Printed Circuit Board) directly and extracting light from the sapphire substrate. Light transmitting electrodes of Ni/Au are replaced with Rh, the material for ohmic contact with higher reflectivity, to recycle light, resulting in enhancing light extraction efficiency. Nitride semiconductor layers are connected to PCB directly through electrode pads, which structures dissipate heat easily, resulting in improving heat dissipation efficiency.
However, flip-chip packaging suggested new problems to semiconductor manufacturers. Backend processing companies which are specialized in the flip-chip packaging technology of packaging, assembly, and test service are in need.
The typical structure of flip-chip packaging extracts light not from top of the device but from sapphire substrate as shown in FIG. 1.
Refractive index of GaN and air is 2.4 and 1 respectively. The noticeable difference between the refractive indexes results in particularly small critical angle of 23°. The small amount of light due to the critical angle can only be extracted to air and the rest of light is captured within LED structure inside by internal total reflection. In addition, top-emission type LED emits light through p type GaN epi layer and hence absorption loss take places by the metal of p type transparent electrodes and p type pad electrodes, resulting in further decreased light emitting efficiency too.
However, in the case of flip-chip LED of mesa type, light paths are formed via not to p metal but to sapphire substrate by shaping reflective layer with the metal of high reflective index such as Ag. As a result, most of light is sufficiently extracted through the thin sapphire substrate processed with lapping and polishing processes. Reflective index difference between 1 of air and 1.76 of a sapphire substrate with good transmittance decreases and the critical angle is larger than the critical angle of GaN epi to air. This means that total internal reflection decreases.
In addition, light absorption loss by p type metal electrodes can be also decreased and hence light extraction efficiency is expected to increase also. This type package also shows excellent heat dissipation performance in comparison with wire bond package in general use; therefore, this type technology is more adaptive to embody high power devices. Besides, the package size can be minimized proportional to chip size, resulting in smaller and lighter devices.